The present invention relates to a method for dividing the volume of a mass of solid fine particles containing Naturally Occurring Radioactive Materials (xe2x80x9cNORMxe2x80x9d) according to activity levels, thereby reducing the amount of material requiring treatment, and the treatment of the material requiring it. The invention is particularly applicable to industries involved in the extraction of hydrocarbons and minerals from the earth where NORM-containing soil and precipitates generated during the extraction, processing, and storage of hydrocarbons and minerals must be treated. More specifically, the invention relates to a method for extracting and/or segregating materials containing NORM components from materials generated during the process of hydrocarbon or mineral recovery. In one application of the invention, NORM is separated from bulk solids, then placed in solution, and injected into a formation similar to the subterranean formation from which the material was originally extracted.
Naturally Occurring Radioactive Materials (xe2x80x9cNORMxe2x80x9d) include isotopes of uranium and thorium, and their decay chain daughters such as radium, radon, and lead. All of them are present in the earth""s crust in immobile and mobile forms. NORM can be released from the earth""s crust due to naturally occurring disturbances or human activity such as, for example, hydrocarbon recovery processes.
From a regulatory standpoint, only Technologically Enhanced Natural Radioactive materials (xe2x80x9cTENRxe2x80x9d) require remediation. Technological enhancement is considered to be the concentration of radioactive substances through a process not intended to produce radioactive substances. As used herein, the term xe2x80x9cNORMxe2x80x9d will be understood to include TENR. In sufficient quantities and concentrations, NORM may present a health risk due to radiation exposure by its increasing the background levels of radiation. Consequently, the disposal of such materials is subject to regulation.
Water recovered or utilized in the process of extracting of hydrocarbons and other materials from substrates almost invariably includes alkaline metal ions of Group I-III elements of the Periodic Table such as cations of barium, strontium, calcium, and magnesium. Also present in the water, along with radioactive substances, are anions such as sulfate, bicarbonate, carbonate, phosphate, fluoride, chloride, and the like. Concentrations of anions and cations in the water, which exceed the solubility of salts formed from the anions and cations, will result in the precipitation of solid salts that may entrap accompanying radioactive substances.
Thus, a problem associated with the emergence of solids in many hydrocarbon and other production operations is the inclusion of NORM in those materials. For instance, NORM species are generated in significant quantities during many oil well drilling and hydrocarbon recovery processes. These NORM materials are present in the earth""s crust as water soluble or insoluble constituents. The majority of water insoluble U238 and Th232 forms of NORM, remains in the earth""s crust or substrate, while waiter soluble NORM materials such as Ra226, Ra228 and R222 may be transported to ground level along with water produced during the oil drilling process.
For example, in the hydrocarbon recovery industry, and in various oil field operations, difficulties are often encountered in dealing with NORM contained in complex form in mineral materials including magnesium sulfate, barium sulfate, strontium sulfate, radium salts and the like. Most notably, the solid, barium sulfate, which has properties making it extremely difficult to treat, retains NORM. The most commonly encountered NORM materials associated with hydrocarbon drilling and recovery processes are isotopes of radium and radon. Uranium and thorium, themselves present in rocks and shales, are insoluble in reservoir fluids, and therefore largely immobile. On the other hand, Radium (226Ra and 228Ra) and Radon (222Rn), radioactive species, components of NORM, are soluble in water.
During oil drilling operations or other hydrocarbon recovery processes, radioactive substances noted may be present in water associated with processes. During the transportation of fluids, these radioactive nucleotides may be encapsulated into a solid matrix. The encapsulation of these radioactive substances occurs with the precipitation of salts from the fluids as the physical or chemical conditions change. Such changes include a decrease in the proportion of water, thus an increase in the concentration of various ions, pressure decrease, and temperature decrease. Precipitation of these salts involves the formation of crystal structures that may encapsulate or entrap radioactive substances as impurities in mineral scales. The radioactive materials are entrapped during the crystal formation process.
Since NORM materials are typically encapsulated in a complex soil or mineral matrix, cost-effective removal and disposal of NORM materials require techniques to penetrate the soil and mineral matrix and extract the radioactive materials prior to release of the solid material. Treating NORM-containing waste material, especially in the case of waste material generated in hydrocarbon recovery, can present additional problems. Often such materials include debris, oil, water, grit, scale and dirt. In some cases, NORM-containing solids are entrained in water-oil emulsions. Thus, the composition of the NORM-containing waste material can present difficulties which must be overcome to treat the material.
Current disposal techniques are directed to treating the entire NORM-containing mass. However it has been found that radioactive substances may, in many cases, be segregated from bulk masses of materials generated, for example, in oil and gas production. It has also been determined that in many such cases NORM-containing materials may be segregated from the bulk of the materials, treated and injected or otherwise disposed of.
Research into the mechanisms for encapsulation and generation of NORM materials has revealed that the distribution of NORM within the solid materials of interest is governed or affected by two factors: (1) physical characteristics such as particle size and (2) the particular type of mineral matrix of the solid materials. In order for NORM to be encapsulated in a precipitate, the matrix must be porous, not an impervious crystal structure. Particle growth, resulting in NORM encapsulation, involves a nucleation process and the type of matrix being formed. A likely location of encapsulated NORM materials is in precipitates such as BaSO4, CaCO3, CaSO4. These materials may be differentiated from, for example, sand, on the basis of various physical characteristics such as density, particle size distribution, and porosity. Further analysis has demonstrated that these forms of solid and mineral matrices can be segregated on the basis of physical characteristics such as particle size distribution and/or density.
It has been discovered that through the use of one or more appropriate solids classification and separation techniques, materials containing higher levels or concentrations of NORM can be separated from the bulk of the materials, thereby reducing the volume of the contaminated, e.g., radioactive, material. In the practice of the invention, it has been found that the majority of contaminated material may be isolated, to form a greatly reduced volume of solid material requiring treatment.
In one embodiment, the invention provides a process for separating a material including a mass of solid fine particles, including particles incorporating NORM. The mass of solid fine particles is classified into samples based upon one or more preselected criteria The level of radioactivity associated with one or more of the samples is determined, enabling separation of the mass into fractions based upon levels of radioactivity associated with the fractions. The term xe2x80x9cfinexe2x80x9d as used herein refers generally to a mass of solid particles having an average particle size which permits release or extraction of radionuclides using an aqueous solution under the conditions and within the time intervals specified herein to the extent that the activity level of the mass of treated particles is below the specified release criterion, typically a 226Ra activity level of 30 pCi/gm. The average size of such particles may generally be in the range of from about 2.5 to about 2500 mesh, and in many cases from about 40 to about 80 mesh and generally less than one inch. Thus, the term xe2x80x9cfinexe2x80x9d as used herein excludes debris such as wood, large rocks or pieces of concrete, broken tools, parts and similar metallic debris, discarded packaging and protective apparel and other similar materials. Classification of the material is the preliminary step that allows for subsequent separation of the material. Classification of the mass of solid fine particles into fractions may be accomplished based upon one or more selected criteria, for example, particle size, particle density, particle hardness, particle composition, particle adhesion, particle cohesion, and other physical characteristics. Classification typically involves sampling the bulk material, and selecting one or more criteria for classification, for example particle size. Classification of the material is normally done on a laboratory or similar small scale. After the material has been classified, the level of radioactivity associated with each samples fraction is determined. This, in turn, allows for possible grouping of the separated sample fractions based upon the level of radioactivity associated with each sample fraction. Separation of the bulk mass of solid fine material into fractions is based upon the sample classification. One or more of the separated fractions of fines may then be selected for further treatment, leaving the remainder of the mass for disposal. By reducing the volume of solid material to be treated, the chemical loading on the treatment facility is significantly reduced, along with the size and capacity of the processing equipment required to treat the material.
In another embodiment, the invention provides a process for releasing or extracting radionuclides from solid fine particles by chemical treatment. In the process, the solid fine particles are contacted with an aqueous solution consisting essentially of water and a chelant in a molar concentration of less than about 0.5 molar, the aqueous solution having a pH of less than 14 and At preferably from about 5 to about 14. As used herein the term xe2x80x9caboutxe2x80x9d is intended to encompass variations within the limits of experimental or measurement accuracy.
The chelant may be a polyamino acid, for example ethylenedieaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) and the molar concentration of the chelant in the solution may be in the range of from 0.01 to 0.5 molar.
In some cases the preferred pH range is from about 12 to about 14; the pH of the solution may be adjusted with an alkali metal hydroxide. Normally the solid fine particles are contacted with the aqueous solution for a period of less than 2 hours. The contact time may range from 30 seconds to 2 hours; in many cases a period of 30 minutes is satisfactory. The fine particles may be contacted with the solution at a temperature of less than 90xc2x0 C., preferably at a temperature between 25xc2x0 C. and 90xc2x0 C. After treatment, the spent solution may be disposed of, for example, by injection into a subterranean formation such as the one from which the materials originated.
The process of the invention may also include the preliminary steps of separating debris from the material, deoiling the material, and grinding the material to reduce the particle size. In summary, the invention provides a means for substantially reducing NORM disposal costs and employing an environmentally friendly technology for NORM remediation.